The answer is D but I am not sure how to get to the answer. How would I go about solving this problem?

12. Lena operates an almond farm. Her marginal costs are MC=5+3CL where Q is the gallons of almonds Lena produces. Lena's farm generates water use externalities, and the marginal external cost can be written as MEC=Q. The market price for a gallon of almonds is 25. What is the tax per gallon of almonds that will get Lena to produce the efficient quantity? 3) 2 b) 3 c) 4 d)5